Calculate daily water intake by weight, activity, climate and health needs. Personalized recommendation backed by IOM and EFSA guidelines. Free, instant.
CalcVita. (2026). Water Intake Calculator — Daily Needs. CalcVita. Retrieved June 4, 2026, from https://calcvita.com/en/calculators/water-intake
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Discover how much water you truly need each day, why the 8-glasses myth persists, and what science actually says about optimal hydration.
Read the full article →This water intake calculator uses a weight-based formula (33 mL/kg/day) recommended by hydration researchers (Popkin et al., Nutrition Reviews 2010). It adjusts for physical activity based on ACSM position stand on exercise and fluid replacement (Sawka et al., 2007), adds climate-based adjustments (Kenefick & Cheuvront, 2012), and accounts for pregnancy and lactation needs per EFSA 2010 guidelines.
A 50 kg person has very different water needs than a 100 kg person. Weight-based formulas (30-35 mL/kg/day) are more accurate than fixed recommendations like "8 glasses a day," which does not account for body size, muscle mass, or metabolic rate.
The widely repeated "8 glasses of water per day" advice has no strong scientific basis. It likely originated from a 1945 US Food and Nutrition Board report that suggested 2.5 liters of total water per day — but noted that most of this comes from food. Your actual needs depend on weight, activity, climate, and health status.
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